Radial tires are gradually being imposed on various markets, notably the market for passenger vehicle tires. This success is due in particular to the endurance, comfort and low rolling resistance qualities that radial tires have to offer.
The main parts of a tire are the tread, the sidewalls and the beads. The beads are intended to come into contact with the rim. In a radial tire, each of the main parts of which the tire is made, namely the tread, the sidewalls and the beads, has functions that are clearly separated from one another, and therefore has a well-known specific makeup.
A radial tire is essentially reinforced by a carcass reinforcement comprising at least one carcass ply set at an angle substantially equal to 90° with respect to the circumferential direction of the tire. This carcass reinforcement is surmounted radially on the outside, and under the tread, by reinforcing plies that form a belt.
A cross-ply tire differs from a radial tire in that there are at least two crossed plies set at angles other than 90° with respect to the circumferential direction of the tire. The plies are said to be “crossed” because the angles are of opposite sign from one ply to the next.
It will be recalled that the circumferential direction of the tire is the direction in a plane perpendicular to the rotation axis of the tire and tangential to the tire belt reinforcement.
Since the emergence of radial tires, certain cross ply tires have also been provided with a belt reinforcement under the tread.
In both these types of tire, the tread, in direct contact with the ground, notably has the function of providing contact with the roadway and needs to adapt to the shape of the ground. The sidewalls for their part absorb the unevennesses of the ground by transmitting the mechanical forces required to support the load of the vehicle and allow it to move.
The belt reinforcement is a reinforcement which, on the one hand, needs to be sufficiently rigid with regard to edge deformations so that the tire can develop the cornering thrust necessary for steering, and transmit torque for traction or for braking and, on the other hand, be very soft in bending, that is to say allow variations in curvature in its plane in order to provide a sufficient area of contact of the tire with the ground.
As a result, the belt reinforcement generally has a composite structure allowing it to offer the required rigidity for a relatively low weight. The belt reinforcement is generally made up of at least two plies set at different angles, comprising reinforcers in the form of cords, coated with rubber. The reinforcer elements are crossed from one ply to the other with respect to the circumferential direction and may or may not be symmetrical with respect to this direction.